The invention is an extension of concepts developed primarily in the field of multiplex spectroscopy. The field of spectroscopy has an extensive body of literature. The description is intended only as a summary with more detail given only for points salient to the present invention. The reader is referred to Wolfgang Demtroder, Laser Spectroscopy: Basic Concepts and Instrumentation, 2nd Edition. Springer Verlag, New York (1982) for a practical description of the topic or Max Born and Emil Wolf, Principles of Optics, 7th Edition, Cambridge University Press (2002) for a rigorous discussion of the topic.
Hadamard spectrometers, which combine features of a dispersive instrument with multiplexing are well described by Martin Harwit (1979). The general design of a Hadamard spectrometer includes an entrance slit, a collimating element, a diffractive element, a spatial mask, and a detector, along with focusing optics and folding mirrors at various points along the optical path. In some designs, the order of the diffracting element and spatial filter are interchanged. Electromagnetic radiation is dispersed into wavebands by the diffractive element and focused onto a spatial filter, which directs some, but not all of the wavebands to a detector. The detector measures the intensity of electromagnetic radiation for a series of different spatial filters and a series of equations is solved to deduce the intensity of each waveband in accordance with a weighting scheme. For a fuller discussion of the topic see Neil J. A. Sloane and Martin Harwit. Masks for Hadamard transform optics, and weighing designs APPLIED OPTICS 15(1) 107-114 (1976).
Early Hadamard instruments, for example that shown in U.S. Pat. No. 3,578,980 (Decker) issued May 18, 1971 generated a series of Hadamard spatial masks by stepwise movement of a master mask. These systems faced problems with mask alignment leading to several technical advances, none of which were wholly satisfactory. A variant on this design was devised by U.S. Pat. No. 3,586,442 (Tripp) issued Jun. 22, 1971 whereby spatially encoded wavebands are incident on the dispersive element a second time so as to undo the dispersion and concentrate the radiation field on a detector. Hadamard systems based on rotating masks were developed. U.S. Pat. No. 6,271,917 (Hagler) issued Aug. 7, 2001 noted the step in transmission of a binary mask produces ringing under Fourier analysis and proposed mask slits with graded transmission.
Hadamard methods have been applied to interferometers to produce a hybrid Fourier Transform spectrometer. In U.S. Pat. No. 4,750,834 issued Jun. 14, 1988 Fateley et al. describe a method placing an electrically alterable mask in the plane of an interference pattern. Fateley et al. also provide a method for reducing the interferogram centre-burst for FTIR spectrometers in U.S. Pat. No. 5,488,474 issued Jan. 30, 1996. The present invention extends these methods to provide improved signal-to-noise.
In U.S. Pat. No. 4,856,897 issued Aug. 15, 1989 Fateley et al. describe a Raman spectrometer based on a Hadamard electro-optical mask and a single detector. The present invention has an objective to improve the signal-to-noise performance of this design.
More recently, Hadamard designs based on masks generated dynamically by the electro-optic effect (liquid crystals) such as in U.S. Pat. No. 5,235,461 (Kirsch) issued Aug. 10, 1993 or electromechanical effect (micromirror arrays) such as in U.S. Pat. No. 5,504,575 issued Apr. 2, 1996 have been proposed. These approaches suffer from several problems. The duty cycle and consequently sampling rate is limited by the transition time for the mask to transition from one defined state to another defined state. In liquid crystal based designs, the contrast between transmissive and absorbing mask regions is less than 10 bits limiting the precision attainable by the instrument. The duty cycle of micromirror based designs is limited by thermal loading. Furthermore, micromirror designs suffer from diffraction and a non-unity packing fraction. Another recent Hadamard variant illuminates a fixed array of Hadamard masks and measures the transmitted pattern with a focal plane array such as in U.S. Pat. No. 5,050,989 (Van Tassel) issued Sep. 24, 1991. This design has the advantage of being mechanically robust with no moving parts, but has the disadvantage of requiring a large focal plane array. In practical terms, this approach is limited to the visible region of the spectrum where silicon based focal plane arrays are inexpensive.
Spatial dimensions can be multiplexed in the same way as spectral dimensions. The general case is spectral imaging, which produces a data cube with spatial and spectral dimensions. Coifman el al describe an apparatus for multi spectral imaging using a mosaic array of filters in U.S. Pat. No. 758,972 issued Sep. 15, 2009. In U.S. Pat. No. 8,345,254 multiplexing is extended even further to amplify otherwise weak signals. The volume of information in a data cube and the requisite processing requirements led to the development of compressive sampling schemes based on the idea that correlation between points in the cube can be used to reduce the number of parameters required to describe the cube. A fuller description is given by McMackin et al in U.S. Pat. No. 8,717,484 issued May 6, 2014.
A key drawback common to all of the Hadamard variants noted above is that even if optical losses are neglected, only half of the EM radiation entering the entrance aperture is received by the detector(s), on average. This limitation is partially overcome by the arrangement shown in U.S. Pat. Nos. 4,615,619 and 4,799,795 (Fateley) issued Oct. 7, 1986 and Jan. 24, 1989, respectively, who proposed using an array of electro-optical filters that can both transmit and reflect EM radiation to generate standard Hadamard masks. Fateley notes in connection with FIG. 5 that both transmitted and reflected radiation can be measured which could in principle improve the effective throughput. However, the implementation given by Fateley provides a modulation of only 50% (from 5% transmission to 55% transmission). Effectively only half of the EM radiation entering the entrance aperture is used. Fateley does not provide any disclosure about how to use the information from a second detector. A further limitation of all the Hadamard variants noted above is that the best spectral resolution achievable is limited by the fixed geometry of the mask element size.
A Hadamard Transform Time-of-Flight Mass Spectrometer was first described by Brock et al (1998). Ions are continuously introduced via an electrospray needle, skimmed, accelerated and collimated. A collimated ion beam is incident on a Bradbury-Nielsen shutter, which either passes the ion beam undeflected toward the detector or deflects the ion beam above and below the beam axis. The apparatus was later modified (Trapp, 2004) by the addition of detectors above and below the beam axis so that both the direct and deflected beams are measured. The modification increased the duty cycle close to 100% and improved the SNR by 29% compared with the earlier version. An improvement of 44% was expected on theoretical grounds. The difference is attributed to imperfect separation of the ion flux contributing to the wrong detector channel. In both versions, the shutter is temporally modulated according to the rows of a Hadamard matrix to pass packets of ions with pseudo random time shifts. Each packet spreads out in the field free zone with the lightest ions traveling the fastest. The detector receives the superposition of time shifted packets as a time sequence for each row. The inverse Hadamard transform is performed to recover the original mass distribution within each packet. In subsequent work Hudgens et al modulated the ion source to produce Hadamard patterns.
Brock, A.; Rodriguez, N.; Zare, N. Hadamard Transform Time-of-Flight Mass Spectroscopy. Anal. Chem., 70, 3735-3741 (1998).
Trapp, O.; Kimmel, J. R.; Yoon, O. K.; Zuleta, I. A.; Fernandez, F. M.; Zare, R. N. Continuous Two Channel Time-of-Flight Mass Spectroscopic Detection of Electrosprayed Ions. Agnew. Chem. Int. Ed. 43, 6541-6544 (2004).
Hudgens, J. W.; Bergeron, D. A Hadamard transform electron ionization time-of-flight mass spectrometer. REVIEW OF SCIENTIFIC INSTRUMENTS 79(1): 014102 (2008).
The disclosures of each of the above references is incorporated herein by reference or can be studied for further details of constructions which can be used herein.